Question about orbits: Let's say I wanted to fly to Venus. . .

[Earl_Snake-Hips_Tucker]

If this has already been answered in a previous thread, I apologize. Also, I have no background in orbital physics, so I might have some misconceptions. Anyway, here goes:

Venus is inside the Earth’s orbit. Firing off a probe toward Venus would mean traveling at some speed to arrive at Venus at whatever time.

However. . . When we add velocity we actually increase the orbit. (The situation of the spacecraft trying to dock comes to mind, and how speed up actually puts you farther behind because it lofts you into a higher orbit). So, before the probe lifts off, it is orbiting the Sun, along with the earth. So it would seem the probe would really be headed away from Venus and toward the orbit of Mars.

There was a long thread some months back on “Why it not be easy to fire something into the Sun” that prompted this question. We have made it to Venus a number of times. What’s the procedure? A Mars flyby?

[DarrenS]

This NASA page describes how Magellan did it.

[st1d]

Depends. As you stated, adding energy to something in orbit (and everything from earth has orbital energy) makes it fly “higher”. Considering that Earth is about 93 million miles from the sun, and the earth orbits the sun in 365 days, you can see that earths speed around the sun is pretty fast.

That in mind, it would take a heck of a rocket firing in opposition to earths orbit to slow it to a speed that would allow it to sink to venus’s orbit. Not only that, but it would take forever to get there. After all you wouldn’t want to slow it to a speed that was too slow (getting it there quickly), or you’d need another huge burst of power to speed it back up to venus’ orbital speed.

The solution, in simple terms, is to send it outward. By carefully calculating orbital patterns, you can send it to mars, have it slingshot around, then make minor burns near earth, then have it slip into orbit around venus. By going to mars, it sort of bleeds speed from the rocket, at which point you can keep the speed slow enough to get to venus without using huge amounts of fuel. (You send it around mars “backwards”, thereby obtaining the loss of energy without using fuel).

The upside is that if you miss, assuming you don’t bury the probe deep into venus, it takes very little energy to redo the trip and try again. The downside is that it’s complicated.

Basically, the same thing is done to get to the outer planets as well, but in reverse. Instead of coming in close to mars or another object (we use the earth itself more often than mars, building the probe’s energy by repeated slingshots, but mars is a better visual aid), and leaving by a distant orbit, you come in far away (diving for the planet), and leave when you’re really close to the atmosphere. This gives you a tremendous speed boost.

The last downside of these slingshots, besides time, is that they are hard on spacecraft. It takes a fraction of time to get an unmanned probe to mars compared to a human, mostly because doing the same to a human would mean turning that person into mush with the extra G’s.

[kniz]

Maybe this will help.

[rowrrbazzle]

Here’s some info from http://www-b.jpl.nasa.gov/basics/bsf4-1.html (which I found via kniz’s link):

To launch a spacecraft from Earth to an inner planet such as Venus using least propellant, its existing solar orbit (as it sits on the launch pad) must be adjusted so that it will take it to Venus. In other words, the spacecraft’s aphelion is already the distance of Earth’s orbit, and the perihelion will be on the orbit of Venus.

This time, the task is to decrease the periapsis (perihelion) of the spacecraft’s present solar orbit. Recall from Chapter 3…

A spacecraft’s periapsis altitude can be lowered by decreasing the spacecraft’s energy at apoapsis.

To achieve this, the spacecraft lifts off of the launch pad, rises above Earth’s atmosphere, and uses its rocket to accelerate opposite the direction of Earth’s revolution around the sun, thereby decreasing its orbital energy while here at apoapsis (aphelion) to the extent that its new orbit will have a perihelion equal to the distance of Venus’s orbit.

Of course the spacecraft will continue going in the same direction as Earth orbits the sun, but a little slower now.

<snip>

An interesting fact to consider is that even though a spacecraft may double its speed as the result of a gravity assist, it feels no acceleration at all. If you were aboard Voyager 2 when it more than doubled its speed with gravity assists in the outer solar system, you would feel only a continuous sense of falling. No acceleration. This is due to the balanced tradeoff of angular momentum brokered by the planet’s – and the spacecraft’s – gravitation.

[sailor]

Nasa (in the link provided by rowrrbazzle) is correct. st1d is mistaken.

[Agback]

G’day

In case anyone is interested in, for example, Googling, the procedure is called a “Hohmann transfer orbit”, and was first described by Walter Hohmann in Die Erreichbarkeit der Himmelskorper, published in 1925.

Regards,
Agback